Method and apparatus for hydrostatically driving a vehicle with each drivable wheel driven by at least one hydraulic motor connected to at least one hydraulic source

ABSTRACT

In a process for hydrostatically driving a vehicle comprising at least two ideal axles (12) of which each has at least one drivable wheel (11), each drivable wheel (11) being driven by at least one hydraulic motor (9) connected to at least one hydraulic source (6), and the driving power being transmitted from a hydraulic motor (9) via a transmission system (10, 35) to the respective wheel (11), the vehicle is to be usable under different performance requirements and is to be produced and operated in a simple and inexpensive manner. This is achieved in that the drivable wheels (11) are initially driven with a first identical ratio of the transmission system (10, 35), while, when a first load limit is reached, the effective drive of the wheels (11) of at least one ideal axle (12) is reduced and, when a second load limit is reached, the ratio of the transmission system (10, 35) is changed, and at least the wheels (11) driven before the first load limit are equally driven. Furthermore, a hydrostatic drive, in particular for performing the process, with at least two ideal axles which have each a drivable wheel (11), each drivable wheel (11) being connected to at least one hydraulic motor (9) connected to at least one hydraulic source (6) and a transmission system (10, 35) being arranged between each hydraulic motor (9) and the associated wheel (11), is suggested for the solution of the above-mentioned technical problem in such a manner that the ratio of the transmission system (10, 35) is variable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for hydrostatically driving avehicle comprising at least two ideal axles which have each at least onedrivable wheel, each drivable wheel being driven by at least onehydraulic motor connected to at least one hydraulic source, and thedriving power of each hydraulic motor being transmitted via atransmission system to the respective wheel.

2. The Related Art

In a process of this type as is known from FR-A-14 33 626, each wheelcan be shifted between three drive gears over a two-step wheel hubgearing. To satisfy different speed and torque demands, this drive islimited to the magnitude of the ratios and the adjustment range of theassociated hydraulic motor. In practice it has been found that thedriving range covered thereby is not sufficient. With a drive designedfor high torques it is normally only possible to implement small drivingspeeds; a drive for high vehicle speed has most of the time inadequatetorque grades.

The same problem is found in a hydrostatically driven wheel hub drivewhich is known from ER-A-13 60 296, and, in which, in addition to twodifferent planet steps, the drive requirements are defined by the powerrange of the driving hydraulic motor.

In a process which is known from WO 91/01899, the drivable wheels ofdifferent axles are driven by transmission systems having differentratios. With a decreasing demand on load, the drive of the axles withthe greatest ratio is decoupled and, when the load demand furtherdecreases, the same is done analogously with the remaining driving axlesuntil in an extreme case the whole drive power is transmitted from theaxle with the smallest ratio. A plurality of axles are disadvantageouslyrequired for a fine adjustment of the power to be transmitted to theground, To achieve high torques and great driving speeds, as many axlesas are possible have to be used in the vehicle with transmission systemsthat are as finely adjusted as possible with respect to one another.This makes the drive more complex and expensive, and the vehicle cannotbe used in some fields of application because of a great number ofaxles.

SUMMARY OF THE INVENTION

The invention is based on the technical problem to provide a process forhydrostatically driving a vehicle of the above-mentioned type, whereinthe vehicle can be used under different performance requirements and canbe produced and operated in a simple and inexpensive manner.

This technical problem is solved according to the invention in that thedrivable wheels are driven with a first identical ratio of thetransmission system before a first load limit is reached, the effectivedrive power of the wheels of at least one ideal axle is reduced when afirst load limit is reached and the ratio of the transmission system ischanged when a second load limit is reached, and the wheels drivenbefore the first load limit are driven with the same ratio.

At the beginning, all wheels transmit the same power portion to theground due to the initial driving of the wheels with a first identicalratio. As a result, a maximum drive power can evenly be transmitted tothe ground under maximum load requirements, e.g., during start, so thatthe vehicle can be accelerated without any slip. If a desired speed has,e.g., been reached at this first ratio, an increased speed can beachieved by reducing the effective drive of at least one ideal axle, ahigh torque being still transmittable at the first ratio to the groundvia the uninterruptedly driven wheels. As a result, the vehicle is alsodriven in this state with a great propulsive power. For example, if asecond speed has been reached, the speed can further be increased bychanging the ratio of the transmission system and by driving alldrivable wheels, the vehicle being again driven by more axles thanbefore the time at which the second load limit was reached. As a result,a lot of power can again be transmitted to the ground without slip via aplurality of axles. When the second load limit is reached, the ratio ofthe transmission system can for example be reduced, so that a higherspeed can be achieved. This process can be repeated once or severaltimes, depending on the changing capacity of the ratio of thetransmission system, so that the most different torques ofdifferentiated adjustment can be transmitted to the ground selectivelywith a single-axle or multi-axle drive.

First and second load limits can here be determined by external loadsacting on the vehicle and by limits defined within a vehicle control. Ahigher speed demand or higher torque load requirements are for examplepossible as load limits. By analogy with the above-mentionedacceleration of a vehicle, the solution according to the invention canalso be employed when the vehicle is decelerated, e.g., by stepwisedownshifting.

Driving speeds of up to more than 70 km/h can be achieved in the processaccording to the invention, so that the drive is suited for highway andsuperhighway travel. At the same time, it is well suited for use inimpassable terrain.

Moreover, operation after the first drive state with reduced effectivedrive of the wheels of at least one ideal axle saves energy and fuel,and higher speeds can be achieved at the same ratio than with the driveof all drivable wheels. This is especially advantageous under soilconditions under which for example the same multi-axle drive is notnecessary or is disadvantageous.

With the solution of the invention, vehicles can produce greatpropulsive forces and manage grades of more than 60%. Even starting fromstandstill is possible in case of such grades.

The effective drive of a driven wheel is reduced in an especiallyadvantageous manner by lowering the displacement volume of the hydraulicmotor. The drive of a wheel can thereby be reduced continuously withoutthe power as applied by the hydraulic sources being changed. A powercompensation with other driven wheels is performed automatically andwithout jerks. The displacement volume is here the sum of all maximumchanges in volume of the pressure chambers that are caused by themovement of the displacement elements during one shaft rotation or adouble stroke. In hydraulic motors, this term is tantamount to theabsorption capacity, and in hydraulic pumps the term is tantamount tothe delivery volume. When axial piston machines are used as hydraulicmotors, the displacement volume is, for example, variable by adjustingthe pivoting angle.

When the ratio of the transmission system is changed, the displacementvolume of the hydraulic motors is substantially set to zero in anadvantageous manner. As a result, a load-free changing or shifting ofthe transmission system is made possible.

The transmission system is especially shifted in steps.

It is suggested that the ratio of the transmission system should bereduced from i=41 to i=6.6. The higher ratio i=41 has turned out to beof special advantage to a great propulsive power in impassable terrainor in case of extreme grades, and the smaller ratio i=6.6 is ofadvantage to the achievement of high speeds in case of all-wheel driveand also in case of only one driven ideal axle. At the smaller ratioi=6.6, desired maximum speeds of more than 70 km/h can be achieved intwo-axle vehicles both with all-wheel drive and with only one drivenaxle. These ratios are for example well adapted to construction andcrane vehicles. Both of the two ratios are well matched for a solutionaccording to the invention, the ratios evenly covering a large drivingspeed range and a large torque load range. Ratio i means the ratio ofinput speed to output speed or of output torque to input torque.

The ratio of the transmission system is advantageously changed from i=12to i=3. These ratios are very well suited for lightweight off-roadvehicles, showing the above-mentioned advantages, especially forvehicles with a weight of up to 2 t.

The ratio of the transmission system is advantageously changed from i=24to i=4. These ratios are especially well suited for driving vehicles ofup to 10 t with the above-mentioned advantages being maintained.

In a variant of the invention, the displacement volume of the hydraulicsource which is designed as a hydraulic pump and driven by a drive motoris substantially equal to zero and is increased upon increase in thespeed of the drive motor. The drive during idling of the drive motor isthereby relieved. When the speed of the drive motor is increased, e.g.by depressing an accelerator pedal, power is transmitted to thehydraulic motors and the wheels. The displacement volume of thehydraulic pump can be increased up to the maximum to achieve maximumpropulsive power of the vehicle.

In a preferred embodiment, the direction of travel is reversed byreversing the hydraulic pump. As a result, the direction of travel canbe reversed by a simple change-over process on a component, i.e. thehydraulic pump.

The hydraulic motors are preferably set to displacement volume zero inthe absence of fluid flow.

The hydraulic motors can especially be set, at least initially, to amaximum displacement volume for acceleration. As a result, maximumtorque power of the wheel can be transmitted as drive power to theground, so that the vehicle is given a maximum propulsive power. Maximumdemands on load are thus satisfied by the drive. If the maximum loadforces have been overcome, the displacement volume can be lowered again.

During acceleration, the displacement volume of the hydraulic motors ispreferably reduced when the hydraulic pump has reached its maximumdisplacement volume. The drive power can continuously be reducedfollowing a maximum start torque.

The displacement volume of a driven hydraulic motor can be set in anespecially advantageous manner at a relatively lower value than itssetpoint portion in the hydraulic pump volume. As a consequence, thepower delivered by the hydraulic pump is not absorbed by thedisplacement volumes of the hydraulic motors, so that an increasedpressure is obtained between each hydraulic motor and the hydraulicpump. The pressure is higher than would be necessary for a constanttravel, so that the excessive torque is, e.g., available for anacceleration of the vehicle.

It is suggested that when a desired driving speed is reached, the speedof the drive motor should be controlled to a desired value by changingthe displacement volumes of hydraulic motors and/or hydraulic pump. Thedrive motor can thus operate within the range of optimum injection,i.e., it runs under optimum exploitation of the fuel and with an optimumwaste gas composition. The desired setpoint speed can, for example, bestored in the electronic drive means and can be aimed at by controllinghydraulic motors and/or hydraulic pump.

A substantially non-wear permanent brake is preferably activated forbraking. The operative brakes of the vehicle that are prone to wear arethereby relieved. This is, for example, of great advantage during a longdownhill drive in which the permanent brake takes over a considerableamount of the braking power. A power-consuming additional circuit is,for example, possible as a non-wear permanent brake.

The displacement volume of an adjustable additional hydraulic pump,which is coupled with the drive motor, can expediently be changed forchanging the braking effect of the permanent brake. The braking actionof the permanent brake is thereby continuously variable. The additionalhydraulic pump pumps, for example, into an additional flow line in whichthe power transmitted by the additional hydraulic pump is consumed. Theadditional hydraulic pump and the additional flow line are heresubstantially resistant to wear.

A pressure limiting valve is preferably adjusted in an additional flowline into which the additional hydraulic pump pumps so as to vary thebraking action of the permanent brake. The pressure limiting valve isadjustable in an easy manner, e.g. via the electronic drive means; theadjustment effects a changed power consumption in the additional flowline, so that the same can simulate different drive resistances to thedrive motor.

In a special manner, the flow delivered by the additional hydraulic pumpcan be guided into the additional flow line during driving and can beguided into an operative flow circuit during operation. The additionalhydraulic pump of the operative flow circuit which acts as a hydraulicsource can thus be used as a permanent brake during driving at the sametime.

In case of slip of a driven wheel, the displacement volume at least ofthe hydraulic motor driven by said wheel is reduced in a preferredembodiment. The respective torque transmitted to the ground is reducedby reducing the displacement volume; in an extreme case, thedisplacement volume of the hydraulic motor is set to zero.

This is of special advantage to a drive slip control of the vehicle. Ifthe drive torque transmitted to a wheel can no longer be transmitted tothe ground and if individual wheels slip partly or entirely (positiveslip), the respective drive torque is reduced via the displacementvolume of the hydraulic motor until the wheel no longer tends to spin.Hence, especially when the vehicle is accelerated, track stability ismaintained.

This process is also of advantage when the vehicle is decelerated byexploiting the drag torque of the drive motor. If the drive drag is sogreat that a wheel tends to lock on the ground (negative slip), thedisplacement volume of the associated hydraulic motor is reduced to suchan extent that the drive motor is reduced and its speed decreases. In anextreme case, the displacement volume of the hydraulic motor is fullyset to zero, i.e. no torque on the wheel, and the drive motor is idling.All of the hydraulic motors can selectively be adjusted, or only thehydraulic motor of the wheel that tends to lock.

The ratio of the difference of circumferential wheel speed and effectivevehicle speed to the circumferential wheel speed is defined as slip.

A reference value is preferably formed from the speeds of the drivenwheels, the speed of each driven wheel is compared with the referencevalue and the displacement volume of the hydraulic motor of this wheelis changed in response to the deviation of a speed of a driven wheelfrom the reference value. The slip of a wheel can easily be detestedthrough this process, for example, by the electronic drive means.Locking of the wheels and spinning of the wheels are arithmeticallydetermined and the displacement volume of the hydraulic motor is thenchanged to reduce slip. The arithmetical mean value can, e.g., be usedas a reference value.

The permanent brake can expediently be deactivated during braking withthe antilocking system. As a result, the permanent brake no longerdevelops any braking action on the drive wheels, so that the antilockingsystem is not impaired in its effect.

During braking with the antilocking system, the drive motor is set toidling as a variant of the invention. This reduces the braking action bythe motor drag on the wheels, so that the antilocking system remainsunaffected in its effect.

During braking with the antilocking system, the displacement volumes ofthe hydraulic pump and/or the hydraulic motors of the driven wheels arepreferably set to zero. This minimizes the drive force of the hydraulicpump and/or the hydraulic motors transmitted onto the wheels, so thatthe antilocking system remains unaffected in its effect.

A displacement volume of the hydraulic motors is possibly assigned to adrive speed. This saves control circuits for determining the state ofthe hydraulic motors, the drive states are predominantly controlled viathe drive motor, for example by an operator adjusting the acceleratorpedal, and the displacement volume of the hydraulic pump is optionallyadjusted.

The process of the invention can be carried out with a hydrostatic drivehaving at least two ideal axles of which each comprises at least onedrivable wheel, each drivable wheel being connected to at least onehydraulic motor connected to at least one hydraulic source, and atransmission system being arranged between each hydraulic motor and theassociated wheel, and the ratio of the transmission system beingvariable.

The transmission system preferably comprises shiftable gear ratios. Incooperation with the adjustable hydraulic motors or the hydraulicsource, it is thereby possible to define various ranges which arecovered by the possibilities of adjustment of the hydraulic motors orthe source at a fixed ratio. These ranges can follow each other or atleast partly overlap by correspondingly choosing the shift steps.

The smallest ratio is advantageously i=6.6. This ratio has turned out tobe advantageous to achieve speeds of more than 70 km/h, for example witha crane vehicle having a conventional drive motor, which speeds make thevehicles suitable for superhighway travel.

The greatest ratio is possibly i=41. This produces a high propulsivepower with a conventional drive motor and hydraulic source for managinggrades in the range of 60% even from a standstill.

A greater ratio is especially i=12, and a smaller ratio is i=3. Theseratios are of advantage to lightweight off-road vehicles, especially upto a weight of 2 t.

In a variation, a greater ratio is i=24, and a smaller ratio is i=4.This combination of ratios is especially advantageous for vehicles up toa weight of 10 t.

The transmission system is preferably fully integrated into the wheelhub. The transmission system can thus be accommodated in a space-savingmanner in the wheel hub, so that the space between the individual wheelsor ideal axles can be exploited for bearing vehicle parts or othervehicle units.

The transmission system can expediently be a two-step planetarytransmission. Transmissions of this type are of a very small and compactstructure, and such a structure can implement gear ratios of the mostdifferent combinations and sizes. Moreover, a planetary transmission isespecially suited as a wheel hub drive.

In a preferred manner, the transmission system comprises a shiftableidling position. The wheels can thereby be separated entirely from theremaining drive, so that a torque-free wheel state can be shiftedindependently of the operative state of the remaining drive. This turnsout to be of great advantage in an antilocking system, an antislipcontrol or a drive-motor drag control.

In a variation, the transmission system has a shiftable lockingposition. Independently of additional brake devices, the vehicle canthus only be braked by shifting the transmission system. When thevehicle is at a standstill, the locking position can be used as aparking brake.

The transmission system can specifically be provided with hydrostaticactuation means for changing the ratio or for shifting purposes. Thehydrostatic actuation means can be coupled with the hydrostatic traveldrive or may have a hydrostatic energy source of their own.

The hydraulic source is advantageously connected via a distributinggearbox to a drive motor. The drive motor drives the hydraulic sourcewhich may, for example, be designed as a hydraulic pump, the powertransmitted from the drive motor being converted by the distributinggearbox into the desired values before the hydraulic source.

The distributing gearbox can specifically be connected to at least oneadditional consumer. Apart from the function as a travel drive, thedrive motor can for example serve as a drive for a tool.

An adjustable hydraulic pump possibly pumps as an additional consumerselectively into an operative flow circuit or into an additional flowline acting as a substantially non-wear permanent brake. The adjustablehydraulic pump can drive a tool as a hydraulic source or can loadinglyact on the drive motor, for example during travel operation, through apower-consuming additional flow line. The load exerted by the additionalflow line brakes the drive motor which, in turn, has a braking effect onthe travel drive due to its drag. This will for example relieve theoperating brake of the vehicle considerably during constant downhilltravel.

The distributing gearbox can expediently be connected to a feed pump ofthe drive circuit. The drive circuit is thereby constantly suppliedautomatically with sufficient hydraulic fluid via the feed pump in therunning state of the drive motor.

The feed pump preferably feeds a directional circuit of the hydraulicsource. This ensures that in the running state of the drive motor thehydraulic source can be constantly switched. The direction of travel ofa vehicle can be changed via the directional circuit.

Embodiments of the invention will be explained hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic circuit diagram of a hydrostatic drive for atwo-axle vehicle;

FIG. 2 shows a schematic circuit diagram of a hydrostatic drive for atwo-axle vehicle with reverse shifting of the hydraulic source andwear-resistant permanent brake;

FIG. 3 is a first semi-section of a two-step planetary transmission withillustrated planet gears; and

FIG. 4 is a second semi-section of a two-step planetary transmission.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a drive motor 1 which is connected to a distributinggearbox 2. The drive motor 1 is connected to an accelerator pedal 3. Itcomprises a starter ring gear 4 which has oriented thereto a tachometer5.

The distributing gearbox 2 is connected on a drive train to anadjustable hydraulic pump 6 with two flow directions and on a seconddrive train to two successively arranged additional hydraulic pumps of aconstant displacement volume and one flow direction. The hydraulic pump6 is connected via hydraulic lines 8 to a total of four hydraulic motors9 having a variable displacement volume and two flow directions. Each ofthe hydraulic motors 9 is coupled via a variable transmission system 10with a wheel 11. Two wheels 11 are respectively located on one idealaxle 12. The hydraulic motors 9 of one respective ideal axle 12 areconnected in parallel with those of the other ideal axle 12, and thehydraulic motors 9 of the same ideal axle 12 are again connected inparallel with each other. Each wheel 11 is individually driven by ahydraulic motor 9.

The hydraulic motors 9, the hydraulic pump 6, the tachometer 5 and theaccelerator pedal 3 are connected via control lines 13 to a control unit14.

In FIG. 2, the drive motor 1 is also connected to the distributinggearbox 2, and a tachometer 5 which is connected via a control line 13to a control unit 14 senses the drive speed of the drive motor 1. Thedistributing gearbox 2 is connected in a first drive train to a feedpump 15 and the hydraulic pump 6 which acts as a hydraulic source. Feedpump 15 and hydraulic pump 6 are connected in series.

The hydraulic pump 6 with the adjustable displacement volume and twoflow directions is connected via hydraulic lines 8 to four hydraulicmotors 9 which are each connected via a transmission system 10 to awheel 11. Two wheels 11 are respectively located on an ideal axle 12.The hydraulic motors 9 of two ideal axles 12 are connected in parallelwith each other, and the hydraulic motors 9 of the same ideal axle 12are also connected in parallel with each other. Each of the hydraulicmotors 9 has an adjustable displacement volume and two flow directions.

Each of hydraulic motors 9 is connected to an adjusting unit 17 foradjusting the displacement volume. The adjusting units 17 are connectedvia control lines 13 to the control unit 14. An output tachometer 18which is connected via a control line 13 to control unit 14 isrespectively arranged between transmission system 10 and hydraulic motor9.

The transmission systems 10 of an ideal axle 12 are connected viahydraulic lines 8 to an electromagnetically adjustable 4/4-portdirectional control valve 19 via which the various ratios of thetransmission system 10 can be selected. At the supply side, each of the4/4-port directional control valves 19 comprises a container feed line20 and a pump feed line 21.

The volume flow produced by the feed pump 15 is guided via a feed line22 and a respective check valve 23 into the supplying or discharginghydraulic line 8 of the hydraulic pump 6. A spring-loaded, infinitelyvariable pressure limiting valve 23 is connected to the feed line 22.

Furthermore, the feed pump 15 is connected via a proportional pressurecontrol valve 25 to a directional circuit 26. In the direction ofcircuit 26, the proportional pressure control valve 25 is followed by anelectromagnetically adjustable 4/3-port directional switching valvewhich operates a double-acting, double spring-loaded cylinder 28. Thecylinder 28 is mechanically connected to the hydraulic pump 6 andcontrols the displacement volume and direction of flow of the hydraulicpump 6. A differential pressure metering plate 29 is connected to theproportional pressure control valve 25.

A progressively adjustable additional hydraulic pump 7 with one flowdirection is arranged on the second drive train of the distributinggearbox 2. The additional hydraulic pump 7 pumps via a 3/2-portdirectional control valve 30 selectively into an operative flow circuit31 or an additional flow line 32. An electromagnetically adjustablepressure limiting valve 33 and a cooler 34 are arranged on theadditional flow line 32. The electromagnetically adjustable pressurelimiting valve 33 is connected via a control line 13 to the control unit14.

A two-step planetary transmission 35 is shown in FIGS. 3 and 4, each insemi-section. The planetary transmission 35 is designed as an integratedwheel hub transmission and has on the exterior a wheel carrier 36 onwhich a wheel 11 can be mounted. At the vehicle side, the planetarytransmission 35 is firmly connected via the hub member 37 to a vehiclemember. The wheel carrier 36 is rotatably supported on the hub member 37via first roller bearings 38. A drive shaft 39 which has an externaltoothing on its one end is centrically arranged in the hub member 37.

The drive shaft 39 serves as a sun of a first planet step with a firstplanet carrier 40 on which a first planet gear 41 is rotatablysupported. The first planet gear 41 engages internally into the driveshaft 39 and externally into a first ring gear 42. The first ring gear42 is rotatably supported via second roller bearings 43 internallyrelative to the first planet carrier 40 and externally relative to thehub member 37.

A first multi-disc clutch 44 is arranged between first planet carrier 40and drive shaft 39. The first multi-disc clutch 44 is loaded by a firstspring 45 and is therefore closed. In the closed state of the firstmulti-disc clutch 44, drive shaft 39, first planet carrier 40 and firstplanet gear 41 are firmly interconnected.

A first actuation chamber 46 can be subjected to pressure via a firstline 47, which extends through the hub member 37, so that the firstspring 45 can be counteracted via a first intermediate member 48 and thefirst multi-disc clutch 44 can be relieved, i.e. it can be opened. Inthis case, the first planet gear 41, the planet carrier 40 and the driveshaft 39 are movable relative to each other.

A second multi-disc clutch 49 is arranged between the first ring gear 42and hub member 37. The second multi-disc clutch 49 is relieved via asecond spring 50 and a second intermediate member 51, so that first ringgear 42 and hub member 37 are rotatable relative to each other, A secondactuation chamber 53 can be subjected to pressure via a second line 52,so that the second spring 50 can be counteracted and the secondmulti-disc clutch 49 can be closed. In this case, first ring gear 42 andhub member 37 are non-rotatably interconnected.

The first planet carrier 40 is interlocked with a sun shaft 54 arrangedin extension of drive shaft 39. Sun shaft 54 is in engagement with asecond planet gear 55 that is rotatably supported on a second planetcarrier 56. The second planet carrier 56 is firmly connected to thewheel carrier 36. The second planet gear 55 externally engages into asecond ring gear 57 formed on the hub member 37.

The first multi-disc clutch 44 is designed with a 1.8 safety factor andthe second multi-disc clutch 49 with a 1.5 safety factor.

The function and operation of the two-step planetary transmission asillustrated in the drawing shall now be explained in more detail in thefollowing:

The drive shaft 39 is, e.g., driven by a hydraulic motor 9, the firstand second actuation chambers 46, 53 being initially without pressure.As a consequence, the first multi-disc clutch 44 is closed and thesecond multi-disc clutch 49 is open. Drive shaft 39, first planetcarrier 40, first planet gear 41, first ring gear 42 and sun shaft 44are non-rotatably interconnected. The drive torque is transmitted to thesecond planet carrier 56 only via the second planet gear 55 which issupported on the stationary second ring gear 57. The carrier transmitsthe torque in an unchanged form via the wheel carrier 36 to a wheel 11to be driven. In this case, a first gear ratio is, for example, i=6.6.

In a second shift state, the two actuation chambers 46, 53 are subjectedto pressure, so that the first multi-disc clutch 44 is opened and thesecond multi-disc clutch 49 is closed. As a consequence, the first ringgear 42 is non-rotatably connected to the stationary hub member 37, anda torque supplied by the drive shaft 39 is transmitted via the firstplanet gear 41, which is supported on the first ring gear 42, via thefirst planet carrier 40 to the sun shaft 54 which, in turn, transmitsthe drive power to the second planet gear 55 supported on the secondring gear 57, and passes the power on to the second planet carrier 56.In this case, the two planet steps which are connected in seriestransmit the drive torque to the second planet carrier and to a wheel 11to be driven. In this case, the gear ratio is, for example, i=41.

When the first actuation chamber 46 is without pressure and the secondactuation chamber 53 is subjected to pressure, the two multi-discclutches 44, 49 are closed and all transmission parts are non-rotatablyinterconnected; the planetary transmission blocks. However, since thetwo multi-disc clutches 44, 49 are designed with different safetyfactors, the multi-disc clutch having the smaller safety factor, i.e. inthis case the second multi-disc clutch 49, will slip against its ownfrictional resistance under very great torque loads. In the blockingposition, this protects the transmission against destruction caused byexcessive torque forces.

When the first actuation chamber 46 is subjected to pressure and thesecond actuation chamber 53 is without pressure, the two multi-discclutches 44, 49 are opened and the planetary transmission is in an idleposition; no torque is transmitted to wheel 11.

The two-step planetary transmission 35 can be used as a transmissionsystem 10 in circuits of FIGS. 1 and 2.

The function and mode of operation of the circuits of a hydrostaticdrive as illustrated in FIGS. 1 and 2 shall now be explained in moredetail:

The drive motor 1 which is, e.g., designed as a diesel engine drives thehydraulic pump 6 via the distributing gearbox 2. The displacement volumeof the hydraulic pump 6 can be adjusted independently of the drive speedof the drive motor 1, for example, by way of an electricallyproportional adjustment. During idling, the hydraulic pump 6 is in thezero position; when the drive speed is increased through the acceleratorpedal 3, the hydraulic pump 6 is automatically swung outwards. Thevehicle accelerates.

At the wheel side, the hydraulic motors 9 are at a displacement volumeof 0 in the absence of fluid flow and are set to a maximum upon thestart of the drive motor 1. In response to the driving speed and to thedrive motor speed, the hydraulic motors 9 reduce their displacementvolume, thereby accelerating the vehicle. To achieve high driving forceson the one hand, e.g., for off-road travel, and to achieve high speedson the other hand, e.g., for superhighway travel, the mechanical gearratio of the transmission system 10 is changed in addition to theadjustment ranges of the hydraulic pump 6 and the hydraulic motors 9. Atthe time of gearshifting, the hydraulic motors 9 are set to adisplacement volume of 0 to shift in an unloaded manner.

Upon starting, the transmission Systems of the drivable wheels 11 areshifted to a high gear ratio, e.g., i=41, and all hydraulic motors 9 areoperated at the same high displacement volume. The vehicle starts with apermanent all-wheel drive and therefore with maximum traction. When afirst driving state is reached, e.g., a maximum speed of about 13 km/h,the displacement volume of the hydraulic motors 9 of an ideal axle 12 isreduced in a continuously variable manner to zero, so that it is onlythe remaining ideal axle 12 that drives. Driving will be performed withthe hydraulic motors 9 of an ideal axle 12 until a second driving stateis reached, i.e., up to a maximum driving speed of 26 km/h.

The transmission systems 10 are then shifted to a second smaller gearratio, e.g., i=6.6., and the hydraulic motors 9 of both ideal axles 12are again driven with the same displacement volume. In this shiftposition, a maximum speed of about 75 km/h can be achieved in theall-wheel drive at excellent traction.

For permanent operation on roads, for example superhighway travel, thedisplacement volume of the hydraulic motors 9 of an ideal axle 12 isagain reduced and set to zero, so that the hydraulic motors of an idealaxle 12 now drive. As a result, a very high efficiency with good fuelconsumption values is achieved at a maximum speed of up to 75 km/h.

These driving states can be repeated as often as wanted, depending onthe number of the gear ratios of the transmission systems 10, or canalso be carried out for reverse driving with reverse delivery flow ofthe hydraulic pump.

The hydrostatic drive is controlled by control unit 14 which measuresthe speeds of the hydraulic motors 9 via the output tachometer 18 andmeasures the drive speed of the drive motor 1 via tachometer 5.Likewise, the displacement volume of the hydraulic motors 9 and of thehydraulic pump 6 can be varied via control unit 14. Likewise, elementswhich are connected via control lines 13, for example the differentialpressure valves 24, 33 or the proportional pressure control valve 25,can be activated. The same is true for all switchable valves 19, 27, 30.

To accelerate the vehicle, the motor speed of the drive motor 1 isincreased by depressing the accelerator pedal 3, so that thedisplacement volume of the hydraulic pump 6 is increased. At thebeginning, the hydraulic motors 9 have maximum displacement volumes, butare set back to smaller displacement volumes in case of maximumdisplacement volumes of the hydraulic pump 6. The displacement volumesof the hydraulic motors 9 are ahead or their setpoint portion of thepump delivery volume to such a degree that the drive motor 1 is reducedwith respect to its desired speed by a specific amount. This produces ahigher pressure than would be required for instantaneous constant travelbetween hydraulic pump 6 and hydraulic motors 9, so that an excessivetorque is available at wheel 11 for acceleration.

If the drive motor 1 is excessively reduced, the advance of thehydraulic motors 9 is slightly reduced, so that the maximum drivingspeed is achieved at a setpoint drive speed of the drive motor 1.

A reduction of the drive motor at every preselected setpoint speedensures that the motor always operates in the range of optimuminjection, i.e., best fuel exploitation and optimum waste gascomposition.

If the accelerator pedal is reduced in a range of 100% to 30% of theload, the vehicle begins to roll and slows down due to drivingresistances. The speed of the drive motor 1 decreases to a new setpointvalue and the hydraulic motors are subsequently adjusted to smallerdisplacement volumes. When the driving speed of the new setpoint speedof the drive motor 1 is reached, the hydraulic pump 6 will again buildup high pressure and the hydraulic motors will again set the drivingspeed to a new setpoint value.

If the accelerator pedal 3 is reduced in a range of from 30% to 0% ofthe load, the driving resistances and the drag torque of the drive motor1 have a braking effect. The displacement volumes of the hydraulicmotors 9 are proportionally increased, so that the drag of drive motor 1is intensified.

If a wheel 11 spins individually or completely, the displacement volumeon the associated hydraulic motor 9 will be reduced until wheel 11 nolonger spins. In an extreme case, the respective hydraulic motor 9 willcompletely be set to zero, i.e no torque at wheel 11.

An average value with which the individual speeds are compared is formedfrom the speeds measured by the output tachometers 18 as a measure ofthe spinning of an individual wheel 11. If a wheel speed deviates fromthe average value excessively, the respective driving torque will bereduced by the control unit 14 by reducing the displacement volume untilthe speed is again matched to the average value. A driving slip controlcan thereby be realized.

By analogy, when a wheel 11 locks, the displacement volume of thehydraulic motors 9 will be reduced until the drag produced by the drivemotor 1 on wheels 11 is so small that wheel 11 no longer tends to lock.This corresponds to a drag torque control of the motor.

Alternatively, it is possible to activate a non-wear permanent brake.Part of the drive power of the drive motor 1 is here passed via thedistributing gearbox 2 to the additional hydraulic pump 7 which pumpsinto the additional flow line 32 during driving. The pressure which thehydraulic pump can build up in the additional flow line 32 can belimited through the pressure limiting valve 33. The cooler 34 dischargesthe produced heat. The drive power which is output via the additionalflow line 32 is varied by adjusting the displacement volume of theadditional hydraulic pump 7. The drive power has an inhibiting effect onthe drive motor 1 and is transmitted via an increased motor drag ontothe hydraulic motors 9. The hydrostatic travel drive is thereby braked.

If braking is performed via the operative brake with the antilockingsystem, the permanent brake is de-energized, e.g. by reducing thedisplacement volume of the additional hydraulic pump 7, and the drivemotor 1 is set to idling. The hydraulic pump 6 and the hydraulic motors9 are both set to zero displacement volume.

To feed the hydraulic lines 8 connected to the hydraulic pump 6constantly with sufficient hydraulic fluid, the feed pump 15 constantlyensures a sufficient pressure level which optimally fills the hydrauliclines 8, namely through the feed line 22 and the check valves 23. Thepressure which can be built up by feed pump 15 in feed line 22 Isadjusted via the pressure limiting valve 24.

The volume flow delivered by the feed pump 15 is measured via thedifferential pressure metering plate 29. The cylinder 28 is operated viathe 4/3-port directional switching valve 27. The flow direction of thepump can be controlled via cylinder 28 and the displacement volume ofthe hydraulic pump 6 can possibly be adjusted as well. The cylinder 28can be controlled via the proportional pressure control valve 25 inresponse to the volume flow measurement of the differential pressuremetering plate 29 in such a manner that the displacement volume of thehydraulic pump 6 is controlled in response to the speed of the drivemotor 1. The volume flow produced by the feed pump 15 and measured viathe differential pressure metering plate 29 is a reference value for thespeed-dependent control.

Optionally, the control unit 14 can exactly assign a displacement volumeof each hydraulic motor 9 to every driving speed. The hydrostatic driveIs here operated with a motor control. This motor control can besuperimposed, at least portionwise, by a corresponding motor control,whereby under specific load requirements and in specific operativestates the drive motor 1 is influenced by regulating the displacementvolumes of the hydraulic motors 9 and/or the hydraulic pump 6.

During operation, the entire drive power of drive motor 1 is transmittedvia the distributing gearbox 2 to the additional hydraulic pump 7. Theadditional hydraulic pump 7 delivers the whole volume flow into theoperative flow circuit 31 by correspondingly switching the 3/2-portdirectional control valve 30. Tools, such as telescopic arm, bucket,hoisting winch, or the like, are operated via the operative flow circuit31. During operation the hydrostatic traveling drive can be entirelyuncoupled from the drive via the distributing gearbox 2. Optionally,this is also achievable through an idle position of the transmissionsystem or through zero position of the displacement volumes of hydraulicpump 6 or hydraulic motors 9.

I claim:
 1. A process for hydrostatically driving a vehicle, the vehiclecomprising at least two ideal axles each of which comprises at least onedrivable wheel, each drivable wheel being driven by at least onehydraulic motor connected to at least one hydraulic source, the drivepower of each hydraulic motor being transmitted via a transmissionsystem to the respective wheel, wherein said process comprises stepsof:before a first load limit is reached, driving said drivable wheelswith a first identical ratio of said transmission system; and when afirst load limit is reached, reducing the effective drive of said wheelsof at least one ideal axle; and when a second load limit is reached,changing the ratio of said transmission system whereby said wheelsdriven before said first load limit are driven with the same ratio.
 2. Aprocess according to claim 1, wherein the effective drive power of adriven wheel is reduced by lowering the displacement volume of theassociated hydraulic motor.
 3. A process according to claim 2, whereinwhen the ratio of said transmission system is changed, the displacementvolumes of said hydraulic motors are substantially set to zero.
 4. Aprocess according to claim 2, wherein said transmission system isshifted in steps.
 5. A process according to either of claims 2 or 4,wherein the ratio of said transmission system is changed from i=41 toi=6.6.
 6. A process according to either of claims 2 or 4, wherein theratio of said transmission system is changed from i=12 to i=3.
 7. Aprocess according to either of claims 2 or 4, wherein the ratio of saidtransmission system is changed from i=24 to i=4.
 8. A process accordingto either of claims 2 or 4, wherein the hydraulic source includes ahydraulic pump driven by a drive motor, and the displacement volume ofsaid hydraulic source is substantially equal to zero during idling ofsaid drive motor and is increased when the drive motor speed isincreased.
 9. A process according to either of claims 1 or 2, whereinthe driving direction is reversed by reversing a hydraulic pump of thehydraulic source.
 10. A process according to either of claims 1 or 2,wherein said hydraulic motors are set to a zero displacement volume inthe absence of fluid flow.
 11. A process according to either of claims 1or 2, wherein said hydraulic motors are at least initially set tomaximum displacement volume for acceleration.
 12. A process according toclaim 1, wherein upon acceleration, the displacement volume of saidhydraulic motors is reduced when a hydraulic pump has reached a maximumdisplacement volume.
 13. A process according to either of claims 1 or 2,wherein the displacement volume of a drivable hydraulic motor is set toa relatively lower level than its setpoint portion in a hydraulic pumpvolume.
 14. A process according to either of claims 1 or 2, wherein,when a desired driving speed is reached, the speed of said drive motoris set to a desired value by changing the displacement volumes of atleast one of the hydraulic motors and a hydraulic pump.
 15. A processaccording to either of claims 1 or 2, wherein a substantially non-wearpermanent brake is activated for braking.
 16. A process according toclaim 15, wherein the displacement volume of an adjustable additionalhydraulic pump coupled with said drive motor is changed for changing thebraking action of said permanent brake.
 17. A process according to claim16, wherein a pressure limiting valve is adjusted in an additional flowline into which said additional hydraulic pump pumps so as to vary thebraking action of said permanent brake.
 18. A process according to claim13, wherein the flow delivered by said additional hydraulic pump isdirected into said additional flow line during travel and is directedinto an operative flow circuit during operation.
 19. A process accordingto either of claims 1 or 2, wherein upon slip of a driven wheel thedisplacement volume of at least the hydraulic motor which drives saidwheel is reduced.
 20. A process according to claim 15, wherein areference value is formed from the speeds of said driven wheels, thespeed of each driven wheel is compared with said reference value, andthe displacement volume of said hydraulic motor of said wheel is changedin response to the deviation of a speed of said driven wheel from saidreference value.
 21. A process according to claim 15, wherein saidpermanent brake is switched off during braking with an antilockingsystem.
 22. A process according to claim 21, wherein said drive motor isset to idling during braking with an antilocking system.
 23. A processaccording to either of claims 1 or 2, wherein the displacement volumesof at least one of a hydraulic pump and hydraulic motor of said drivenwheels are set to zero during braking with an antilocking system.
 24. Aprocess according to either of claims 1 or 2, wherein a displacementvolume of said hydraulic motors is assigned to a vehicle speed.
 25. Ahydrostatic drive for performing the process according to claim 1, thehydrostatic drive comprising: at least two ideal axles each of whichcomprises at least one drivable wheel, each drivable wheel beingconnected to at least one hydraulic motor connected to at least onehydraulic source, and a transmission system arranged between each ofsaid hydraulic motors and the associated wheel, wherein the ratio ofsaid transmission system is variable.
 26. A hydrostatic drive accordingto claim 25, wherein said transmission system has shiftable ratios. 27.A hydrostatic drive according to either of claims 25 or 26, wherein thesmallest ratio of the transmission system is i=6.6.
 28. A hydrostaticdrive according to claim 27, wherein the greatest ratio of thetransmission system is i=41.
 29. A hydrostatic drive according to eitherof claims 25 or 26, wherein a greater ratio of the transmission systemis i=12 and a smaller ratio of the transmission system is i=3.
 30. Ahydrostatic drive according to either of claims 15 or 26, wherein agreater ratio of the transmission system is i=24 and a smaller ratio ofthe transmission system is i=4.
 31. A hydrostatic drive according toeither of claims 25 or 26, wherein said transmission system is fullyintegrated into a wheel hub.
 32. A hydrostatic drive according to claim31, wherein said transmission system is a two-step planetarytransmission.
 33. A hydrostatic drive according to either of claims 25or 26, wherein said transmission system has a shiftable idle position.34. A hydrostatic drive according to either of claims 25 or 26, whereinsaid transmission system has a shiftable blocking position.
 35. Ahydrostatic drive according to either of claims 25 or 26, wherein saidtransmission system is provided with hydrostatic actuation means forvarying the ratio or for shifting.
 36. A hydrostatic drive according toeither of claims 25 or 26, wherein said hydraulic source is connectedvia a distribution gearbox to a drive motor.
 37. A hydrostatic driveaccording to claim 36, wherein said distribution gearbox is connected toat least one additional consumer.
 38. A hydrostatic drive according toclaim 37, wherein an adjustable hydraulic pump is provided as anadditional consumer and pumps selectively into an operative flow circuitor into an additional flow line acting as a substantially non-wearpermanent brake.
 39. A hydrostatic drive according to claim 36, whereinsaid distribution gearbox is connected to a feed pump of the drivecircuit.
 40. A hydrostatic drive according to claim 39, wherein saidfeed pump feeds a directional circuit of said hydraulic source.
 41. Aprocess according to claim 16, wherein said permanent brake is switchedoff during braking with an antilocking system.